Plastics are becoming increasingly important in the construction of automobiles, trucks and off-the-road vehicles. The materials most commonly used are resins, such as polyesters, epoxies and phenolics, which are often reinforced with fibers, particularly glass fibers. Typically, a mixture of resins and fibers is formed into a mass called bulk molding compound (BMC) or a sheet called sheet molding compound (SMC). SMC or BMC is then compression molded to form the desired part.
Plastic parts have many advantages. They are strong, lightweight and will not rust. However, they also have some serious disadvantages, e.g., it is difficult to obtain finish coatings with excellent appearance on the plastic parts. A particularly troublesome appearance problem is "popping" which is an eruption in the coating which develops during the heat curing cycle of the finish coating.
Plastic parts, particularly of the fiber glass-reinforced polyester type, are known to be porous and to contain entrapped air and other volatile materials. These volatile materials are believed to release during the curing cycle of subsequently applied coatings and burst through the coating as it is curing. Primers can be applied to fiber glass-reinforced plastic parts for the purpose of sealing off its porosity. However, it has been found that many of the commercially available primers are simply not effective sealers in that popping still occurs in a subsequently applied finish coat during its curing cycle.